The invention relates to a calculation method to be used in a radio system comprising a set of transmitters and a set of receivers, in which radio system the time difference between the reception moments of the signals transmitted by transmitters is measured, and the geometrical time difference of the transmitted signals is calculated.
In known radio systems, the reception moment of a signal arriving from a transmitter at a receiver helps to determine the location of a subscriber terminal in a radio system. In a prior art method, reception moments of signals transmitted by transmitters, e.g. base stations, are measured with respect to each other. This measured time difference of the signals is also called an Observed Time Difference (OTD).
Generally, transmitters are synchronised in such a way that lower levels of a radio system are synchronised by a synchronising signal coming from an upper level of the radio system. If base stations transmit a signal synchronously, the observed time difference is obtained solely by examining the distances of the base stations. The observed time difference of two synchronously functioning transmitters can thus be calculated by means of a so-called Geometrical Time Difference (GTD). If the locations of the transmitters are fixed and known in advance, the actual location of the subscriber terminal in the radio system can easily be calculated by means of propagation times of the signals. In practice, however, base stations functioning as transmitters do not totally synchronise with each other, and so errors occur when determining locations of subscriber terminals.
In known radio systems, the Real Time Difference of a radio transmitter, i.e. the time difference between transmission times of two transmitters, can be determined by the GTD calculated on the basis of the locations of transmitters and receivers, and by the OTD measured by receivers. The geometrical time difference between two signals is determined by means of the difference in the distance travelled by both signals, and by means of the propagation velocity of the signal. In known radio systems, locating a subscriber terminal is based on the signals transmitted by transmitters and received by receivers. On the basis of signals received by a receiver, information is obtained on the mutual synchronisation of the transmitters. Practically, the known radio systems comprise various receivers, and it is thus complicated to process time difference information based on the signals received by the receivers in such a way that the real time difference and the synchronisation between the various transmitters could easily be detected.
It is thus an object of the present invention to provide a method and an apparatus implementing the method, in such a way that the above problems can be eliminated. This is achieved by a type of method disclosed in the introduction, which is characterized by forming the real time difference between the transmission moments of signals transmitted by a transmitter serving a respective receiver and by the neighbour transmitter of said transmitter as a function of time by means of measuring the time difference between the reception moments of the signals, and by means of the geometrical time difference, in such a way that the geometrical time difference is added to the time difference based on the reception moments, summing up directly measured real time differences in order to calculate the real time differences between the other transmitters in the coverage area of the receiver, calculating the real time differences between the transmitters in coverage areas of various receivers are calculated by making use of the time differences that already have been calculated between transmitters.
The invention also relates to a radio system comprising a set of transmitters, a set of receivers and a measuring means, which measure the time difference between the reception moments of signals transmitted by the transmitters while the receiver is receiving signals, and which calculate the geometrical time difference of the signals transmitted by the transmitters.
The radio system is characterized by comprising a calculating means, which form the real time difference between the transmission moments of signals transmitted by transmitters as a function of time by means of measuring the time difference of signals received by a receiver, and by means of the geometrical time difference, in such a way that the calculating means add the geometrical time difference to the time difference based on the reception moments, the calculating means sum up directly measured real time differences and thereby calculate the real time differences between the other transmitters in the coverage area of the receiver, and the calculating means calculate the real time differences of the transmitters in the coverage areas of various receivers by making use of the time differences that already have been calculated between the transmitters.
The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on measuring differences between transmission times of transmitters, on the basis of which the real time difference is determined also between transmitters which transmit a signal to various receivers.
The method and system of the invention provide a plurality of advantages. In the method of the invention, vectors are formed on the basis of time difference values, which can thus be combined easily. Further, the method utilises a calculating algorithm, which enables the quick determination of the time difference values between all the transmitters in the radio network. The method can easily be implemented e.g. by means of software. By means of the method the synchronisation differences between the transmitters can be determined.